Conventionally, in manufacturing a grain-oriented electromagnetic steel sheet having a high magnetic flux density and excellent in iron loss, a steel sheet is subjected to processing where it is kept at a temperature within a 50° C. to 350° C. range for one minute or more between passes of cold-rolling. Such processing is called inter-pass aging and is described in a patent document 1.
Rolling using a tandem mill has a difficulty in yielding an effect equivalent to that of the inter-pass aging. Therefore, in manufacturing a grain-oriented electromagnetic steel sheet excellent in orientation and high in magnetic flux density, cold-rolling using a reverse rolling mill is generally performed. This is because it is easy to keep the temperature between passes.
A grain-oriented electromagnetic steel sheet high in magnetic flux density contains 3% silicon or more for realizing a low iron loss and is very brittle. Therefore, an edge crack is likely to occur during the manufacture. Further, the edge crack, even if only a small, sometimes becomes larger to cause a sheet fracture. Especially in rolling using a single-stand reverse rolling mill, since the structure of a rolling mill necessitates a work of winding an end portion of a hot-rolled coil around a tension reel, it is highly possible that the steel sheet finally fractures due to a bending stress generated when the coil end portion is wound around the tension reel.
Here, cold-rolling using a single-stand reverse rolling mill will be described. FIG. 4A to FIG. 4E are views showing a cold-rolling method using a single-stand reverse rolling mill in order of processes.
In a cold-rolling facility using a single-stand reverse rolling mill, a rolling stand (reverse rolling mill) 21 is disposed at the center. Further, across the rolling stand 21, a coil leading end-side tension reel 22 is disposed on one side, and a coil tail end-side tension reel 23 and a pay-off reel 24 are disposed on the other side.
Prior to the cold-rolling, a steel sheet coil (hot-rolled coil) 25, which is made by coiling a steel sheet 26 being a target of rolling, is carried to the pay-off reel 24, as illustrated in FIG. 4A. Next, a leading end of the steel sheet 26 is drawn out from the steel sheet coil 25 to be wound around the tension reel 22 via the rolling stand 21.
Thereafter, as illustrated in FIG. 4B, the steel sheet 26 is rolled in a first pass while being given a tension between the pay-off reel 24 and the tension reel 22. Then, as illustrated in FIG. 4C, when a tail end 27 of the steel sheet coil 25 is apart from the pay-off reel 24, the rolling is finished, and as illustrated in FIG. 4D, the tail end 27 is wound around the coil tail end-side tension reel 23 located between the pay-off reel 24 and the rolling stand 21. Thereafter, as illustrated in FIG. 4E, the steel sheet 26 is rolled in second and subsequent passes while being given a tension between the both tension reels 22 and 23.
In the cold-rolling by this method, an unrolled portion 28 is left in the tail end 27 after the first-pass rolling, as illustrated in FIG. 5. Therefore, when the tail end 27 is wound around the tension reel 23, a portion with a certain length of a first-pass rolled portion 30 is wound after the unrolled portion 28 is wound. At this time, a high-curvature portion that is first wound sometimes fractures.
Further, as a result of the first-pass rolling, there is formed a roll bite portion (first-pass roll bite portion) 29, whose thickness changes from t0 obtained after the hot rolling to a thickness t1 obtained after the first-pass rolling. The roll bite portion 29 is also a boundary region between the unrolled portion 28, which has a large thickness and a large bending stress, and the first-pass rolled portion 30, which has undergone work hardening. Therefore, the roll bite portion 29 sometimes suffers fracture when it is wound.
Therefore, from a viewpoint of productivity improvement, it is important to alleviate brittleness of a material to prevent the occurrence of sheet fracture. Such sheet fracture sometimes occurs not only in a grain-oriented electromagnetic steel sheet having a high Si content but also when other brittle steel sheet (for example, a steel sheet of high-carbon steel) is rolled in the above-described manner.
A patent document 2 describes an art to alleviate brittleness of a material when a brittle steel sheet such as an electromagnetic steel sheet is cold-rolled. In this art, at the time of the cold-rolling using a continuous tandem rolling mill, by setting a strip temperature to 50° C. to 150° C. in advance, a steel sheet is heated before carried to a first rolling stand, so that, between rolling stands, the steel sheet is kept at a temperature within a predetermined range.
However, applying this art to a reverse rolling mill gives rise to the following problems.
(i) In the rolling using the reverse rolling mill, since a tail end is wound around a tension roll after first-pass rolling is completed, the effect of heating a steel sheet, even if performed beforehand, is weakened before the winding.
(ii) Since the rolling is stopped at the first roll bite portion in spite that this portion is a portion most likely to fracture, it is not possible to obtain sufficient deformation heating.
(iii) After being exposed to rolling oil, the first-pass roll bite portion is exposed to the outside air until it is wound around the tension reel and thus is rapidly deprived of heat when the rolling oil vaporizes.
(iv) In rolling the grain-oriented electromagnetic sheet, if a coil before being cold-rolled is heated to a temperature that is increased in consideration of an amount of the deprived heat, the temperature becomes too high, so that a magnetic characteristic of a finally obtained steel sheet deteriorates.
Therefore, even applying the art of the patent document 2 to the reverse rolling mill cannot produce a sufficient effect of alleviating brittleness when the tail end of the steel sheet is wound around the coil tail end-side tension reel.
Further, a patent document 3 describes an art to prevent a decrease in temperature of a steel sheet by covering an area between a pay-off reel and a rolling stand by a heat-insulating enclosure wall. It is conceivable to solve the problem (iii) of the patent document 2 by using this art.
In this case, however, the heat-insulating enclosure wall needs to cover a range up to an area close to the rolling stand. In the reverse rolling mill, the tail end side changes to a leading side in even-numbered passes. Therefore, a large volume of accompanying fume enters the inside of the enclosure wall and the fume is filled inside the enclosure wall, which makes it difficult to ensure measurement precision of instrumentation devices (a sheet-thickness gauge, a sheet-temperature gauge, and the like) inside the enclosure wall and to ensure the maintenance of a facility.
Further, increasing a reel diameter in order to reduce the bending stress itself could reduce the occurrence of the sheet fracture, but applying the increase in the reel diameter to existing devices is difficult because of space. Further, an unrolled portion becomes longer by the increased size, which lowers yields.
Patent document 1: Japanese Examined Patent Publication No. Sho 54-13846
Patent document 2: Japanese Patent Application Laid-open No. Sho 61-132205
Patent document 3: Japanese Patent Application Laid-open No. Sho 61-135407